Tumor cell and viral vaccines successfully prevented transplantable and carcinogen-induced cancers in rats. Passive immunity processed as murine-specific IgG successfully prevented spontaneous leukemias and chemically (3-MC)-induced tumors in mice. Endogenous retroviruses derived in high titer (10 to the 11th power virus particles per ml) from various strains of mice (AT124, M55, C57L) and grown in human and canine cells, and baboon virus (grown in canine and human cells) administered intravenously or intraperitoneally successfully eliminated existing cancer, including multiple metastatic cancers in dogs and (preliminary) in cats. This radical new approach to viral immunotherapy of a variety of cancers in dogs and cats will be extended to rabbits and primates bearing cancers.